1. Field of the Invention
The present invention relates to a focusing state signal output apparatus applied to, for example a microscope, for outputting a focusing state signal, which represents a focusing state corresponding to a relative distance between an objective of the microscope and a sample.
2. Description of the Related Art
Currently, a microscope able to observe a microscopic sample and further to record the observed image as a video image is widely utilized extending from researches in the biological field to inspection processes in the industrial field. In using this kind of microscope, focusing operation is generally performed by adjusting the focus of the sample through operating a focusing handle.
However, in the case where, for example, an objective with high magnification (hereinafter, also called a “high magnification objective”) is used, it is difficult to perform promptly the focusing operation because the high magnification objective has a narrow range of focusing due to its small depth of focus. Therefore, there has been a problem that the focusing operation needs considerable skill resulting in fatigue of an operator and decrease in production efficiency. It is very important to solve this problem for reducing inspection time through performing the prompt focusing operation particularly in a routine work such as an inspection process.
Then, various kinds of automatic focusing (AF) apparatus used for a microscope, which make the automatic focusing operation possible, have been proposed. In addition, a variety of apparatus aiming improvement of the AF apparatus have also been proposed.
For example, for the AF apparatus used in the industrial field, the following items have been required in addition to enhancement in the operational easiness and the throughput described above. For example:
(1) Detecting and measuring all the defects in each layer and all the line widths between patterns in the case of a sample with bumps on the surface such as those in a multi-layered semiconductor wafer; and
(2) Measuring microscopic bumps on the sample with high precision.
Accordingly, an apparatus having performance suitable to these inspection and measurement has been proposed.
In the AF apparatus used in the industrial field, because of availability for a sample, reduction of AF time, etc. the apparatus of the so-called active AF type where a light beam such as infrared laser is projected on the sample and then the focusing operation is performed detecting a state of the reflected light beam prevails.
On the other hand, in the AF apparatus used in the biological field, because of requirement for more precise focusing, application to a transmission type sample with too low reflectivity to be detected by the active AF type, etc. the apparatus of the so-called passive AF type where the AF operation is performed detecting a contrast of the observed image is used.
Now, a conventional AF apparatus of both the active type and the passive type comprises, as one body, a focusing state signal output unit and a focusing portion in order to keep the above performance. Further, the focusing state signal output unit outputs a focusing state signal, which represents the focusing state corresponding to a relative distance between an objective of a microscope and a sample. The focusing portion focuses by adjusting the relative distance between the objective and the sample according to the signal from the focusing state signal output unit.
However, recently particular AF functions have been required in optical equipment used in various inspection devices or systems. Most of the optical equipment drive, by part of particular manners, a stage for mounting the sample, a revolver for loading the objective, or the like, which compose the focusing portion.
In order to realize the stable AF function independently of a sort or a driving method of the focusing portion, grown high is a need that only focusing state signal output units are separately prepared and each apparatus manufacturer develops the apparatus for performing AF operation with the signal from the focusing state signal output unit corresponding to a driving mechanism of each focusing portion, namely a need for a separate “focusing state signal output apparatus” as the so-called OEM product.
In this situation, the most necessary requirement for the focusing state signal output unit is to realize the precise AF operation by part of outputting the signal, the so-called error signal which represents the focusing state of the sample, under the optimal states for each focusing mechanism developed by each apparatus manufacturer.
The following device is disclosed. The device employs the manner where a plurality of detection part for such an error signal are prepared and the best signal is selected corresponding to each situation (see JP 10-260363, A).
In the above device, each photoreceptor sensor of a pair of photoreceptor sensors is divided onto 3 (concentric) light-receptive surfaces. For each laser beam incident on each light-receptive surface, error signals are calculated on the basis of plural combinations of the light-receptive surfaces. Then, the optimal (a curve having a good characteristic) combination of the light-receptive surfaces is selected from a plurality of the calculated error signals depending upon the pupil diameter of the objective. Thereby, it becomes possible to perform the stable AF operation.
In the above-mentioned manner, however, the error signals are beforehand adjusted mechanically and are selected in an alternative way from the prescribed combination of the photoreceptor devices. Therefore, it is impossible to change the shape of the error signal arbitrarily. Further, since there is no idea of outputting the error signals to the external device, such an idea as to make AF operation managed by an external system is not thought out therein. Still further, it is difficult to freely customize the signals from an apparatus consisting of a complicated and precise optical system. Consequently, it is impossible to keep the sufficient performance through the mechanical selection of the error signals considering versatile availability for OEM, etc.